“This was a heinous and cowardly act, and given what we now know about what took place, the FBI is investigating it as an act of terrorism,” the president said Tuesday from the White House briefing room. “Any time bombs are used to target innocent civilians, it is an act of terror.”

On Monday evening, in his first statement on the explosions, Mr. Obama didn’t call the attack “terror” or “terrorism;” he called it a “tragedy,” sparking criticism from some conservatives that he was underplaying the meaning of what had happened. Yet right after the president’s remarks, a White House official told reporters that the administration was treating the attack as terrorism.

“Any event with multiple explosive devices – as this appears to be – is clearly an act of terror, and will be approached as an act of terror,” said the official, according to Politico. “However, we don’t yet know who carried out this attack, and a thorough investigation will have to determine whether it was planned and carried out by a terrorist group, foreign or domestic.”

Three people died and 176 were injured in two bomb explosions at 2:50 p.m. Eastern time near the finish line of the storied Boston Marathon.

By Tuesday morning, when Obama made his second public statement, he acknowledged that he had no answers – who carried out the attack, why they did it, whether it was the act of a terrorist organization, foreign or domestic, or the act of a “malevolent individual.”

“Clearly we're at the beginning of our investigation,” he said. “It will take time to follow every lead and determine what happened. But we will find out. We will find whoever harmed our citizens, and we will bring them to justice.”

Since news of the explosions broke Monday afternoon, Obama has blended speed with caution and a tone of resolve as he has sought to reassure the nation. He spoke in the briefing room a little over three hours after the attacks, not because he had answers but to convey that his administration and all levels of law enforcement were on the case.

The president’s initial hesitance to call the attack terror or terrorism was “classic Obama,” says Martin Medhurst, an expert on political rhetoric at Baylor University in Waco, Texas.

“It tells us he’s a cautious speaker,” says Mr. Medhurst. “But we’ve known that about Obama for a long time. It didn’t surprise me that he wouldn’t immediately call it terror.”

Obama has come under criticism in the past for his reactions to actual or attempted terrorism, both over the speed of his response and the words he has used. On Christmas Day 2009, a Nigerian with explosives hidden in his underwear attempted to bomb a Delta Airlines flight to Detroit. Obama was criticized for waiting three days before responding publicly to the failed attack.

Last September, when the US consulate in Benghazi, Libya, came under attack – resulting in the deaths of four Americans, including the US ambassador to Libya – Obama was initially vague about whether he considered the attack terrorism. Obama faced criticism, and the episode became an issue in the presidential campaign.

But the history books are full of examples where government officials and the media have not exercised caution in putting out preliminary – and ultimately false – information about who might have perpetrated an attack on Americans. When a federal building was bombed in Oklahoma City in 1995, initial suspicion focused on a Jordanian-American man, who was arrested and interrogated. Soon it became clear the attack was a case of domestic terrorism.

A year later, during the Summer Olympics in Atlanta, a bomb was set off in a park, killing two people. A security guard on the scene named Richard Jewell became the prime FBI suspect in the case, and was hounded by the media. Mr. Jewell was eventually exonerated, after investigators determined that the perpetrator was domestic terrorist Eric Rudolph.